Coordinated Multi-Point (CoMP) transmission/reception has been proposed as a promising technology to meet the 3GPP (Third Generation Partnership Project) LTE-Advanced (LTE-A) requirements by improving performance of cell-edge user equipment (UEs) in particular. In CoMP operation, multiple transmission/reception points cooperatively transmit to or receive from one or more UEs to improve performance, especially for those UEs that would otherwise, in the case of downlink, see significant interference from some transmission points if the transmission points do not cooperate. A transmission point (TP), termed from the perspective of downlink, generally refers to a radio unit controlled by the scheduler in a base station (referred to as an eNodeB or eNB in LTE). A base station may control a single TP, in which case the TP is the same as a base station or an eNB. In this case, the CoMP operation refers to the case that there is coordination among eNBs. In another network architecture, a base station or eNB may control multiple transmission points (TPs), which often are referred to as radio units or radio heads. In this case, coordination among TPs will happen naturally, and is easier to achieve since they are controlled by a centralized scheduler within the eNB.
In general, CoMP techniques refer to a broad range of coordination mechanisms including interference avoidance. One such technique is joint-transmission where antennas from two or more TPs are used together in a multi-antenna multi-input multi-output (MIMO) transmission to a UE. More generally, one can consider distributed antenna type of deployments where a transmission to a terminal may be from antennas distributed geographically. Clearly, the difference from a conventional MIMO operation is that the antennas are not necessarily co-located.
In some network deployments, TPs may be co-located, in which case it is feasible to connect them to a single eNB. An example is the well-known three-sector/cell deployment where a single eNB has three service areas, referred to as sectors or cells. In some other deployments, TPs may be geographically separated, in which case they can be controlled by either separate eNBs or a single eNB. In the former case, TPs are typically under the control of separated schedulers that may coordinate in a peer-to-peer fashion. Different types of eNBs with possibly different transmission powers constitute a so-called heterogeneous network. In the case of geographically separated TPs controlled by a single eNB, the TPs, often referred to as remote radio units (RRUs) or remote radio heads (RRHs), connect to a single eNB via optical fiber, and a centralized scheduler controls/coordinates all the TPs.
Each TP, whether co-located or geographically separated, may form its own logical cell, or multiple TPs may form a single logical cell. From a user equipment (UE) perspective, a cell is defined as a logical entity that a UE receives data from and transmits data to, in other words, “serves” the UE. The cell that serves a UE is called the “serving cell.” The geographic area covered by the logical entity sometimes also is referred to as a cell, such as when a cell-edge UE is mentioned to describe a UE located at the edge of the coverage area. A cell usually has an associated cell identifier (cell-ID). A cell-ID is typically used to specify the pilot signals (also referred to as reference signals) that may be unique to the cell and scramble the data transmitted to the UEs “attached” to, that is, served by, that cell.
In conventional non-CoMP multi-antenna (MIMO) operation, a single TP, which is the serving cell of a UE, adapts the transmission parameters based on the quality of the link to the UE. In this so-called “link adaptation” as commonly adopted in modern wireless communications, a UE needs to estimate a channel quality of a hypothetical data transmission which is traditionally from a single cell for non-CoMP operation. Channel quality is often represented as a modulation and coding scheme (MCS) that could be received by the UE with an error probability not exceeding a particular threshold. The UE may also feed back some recommendation of spatial transmission parameters, such as transmission rank indication (RI), precoding matrix index (PMI), and the like. In CoMP operation, the transmission from multiple points also needs to adapt to the link condition as seen by the UE.
The UE relies on pilot signals (also known as reference signals (RSs)) sent from a serving cell for channel estimation and for channel quality measurements that are reported back to the eNB. Often the reference signals are scrambled with a sequence specific to a cell-ID of that particular serving cell. In order to estimate a channel and to make channel quality measurements, the eNB must have a mechanism that enables the UE to estimate the channel and also measure the interference. The usual mechanism to enable the channel estimation by the UE is for the eNB to send pilot signals from each of the transmit antennas, which essentially sound the channel. A pilot signal is a waveform or sequence known by both the transmitter and receiver. In OFDMA systems, the pilot signals usually correspond to a pilot sequence on a set of time-frequency resource elements (REs) within a time/frequency grid, where a resource element is a subcarrier in OFDM transmission. The UE would then use the pilot signals to compute channel estimates at each subcarrier location by performing interpolation and noise suppression, and to measure a channel quality. Further pilot signals are also needed at the UE to construct the “effective” channel for purpose of coherent demodulation. An effective channel, corresponding to one or more data streams or layers of a UE, is the precoded/beam-formed channel that a UE's receiver effectively sees applied to a data modulation signal at the receiver.
In Releases 8 and 9 of the 3GPP LTE standards, Common or Cell-Specific reference signals (CRS) (and, in Release 10, Channel State Information Reference Signals (CSI-RSs)), corresponding to a set of CRS ports (CSI-RS ports in Release 10), are sent from an eNB and are intended for all UEs in a cell served by the eNB. The CRS ports could correspond to the set of physical antennas at an eNB or a set of virtualized antennas observable at all UEs served by the eNB. These RSs may be used for channel estimation for channel quality and/or for spatial feedback measurements. A UE can compute and report a recommended PMI from a pre-defined codebook, as well as providing associated RI and CQI (Channel Quality Information, or Indication) feedback, for maximizing the total rate of transmission (or sum CQI) at the UE.
Broadly, joint processing (JP) schemes refer to either i) Joint Transmission (JT) (where data is transmitted to a UE from two or more TPs) or ii) Dynamic Point Selection (DPS) (where data is dynamically transmitted from one of the two or more TPs). The term joint processing refers to the fact that the two TPs should be able to process the data intended for a UE at any time. Further, if at least one data stream is sent simultaneously from two or more TPs, it is referred to as a coherent joint transmission (which requires some phase alignment) and if independent data streams are sent from each TP, it is referred to as non-coherent JT.
Timing Issue
The LTE system is primarily designed, and test cases were setup, with the implicit assumption that the antenna ports represented by the CRS/CSI-RS ports are co-located. Typically in those cases, the individual antennas can be assumed to be calibrated. Accordingly, the codebooks and the CSI feedback approaches are defined based on these implicit behaviors. However, in a CoMP communication system, RRUs/RRHs, and corresponding antenna ports, chosen for transmission (for example, the two closest RRUs/RRHs) to a UE may have different path losses. That is, the signals from each RRU/RRH/antenna port may propagate over a completely different path and/or the UE may be much closer to one RRU/RRH/antenna port than the other, with the result that the UE may see a much larger time delay from one chosen antenna port than another chosen antenna port. This is especially true in the case of small cell or indoor deployments, where a UE may come very close to one of the antennas. Such time delay may introduce frequency selective phase rotation between groups of antennas from different eNBs, in which case coherent joint MIMO transmission from non-colocated groups of antennas may be challenging.
Feedback Overhead
Further, in these systems, to support joint transmission a UE has to feedback the CSI information assuming joint transmission from the aggregated set of antennas corresponding to the transmission points. Such CSI could include information related to the transmission rank, which is essentially the number of spatial streams transmitted to a UE, the channel quality index information, which is essentially the modulation and coding scheme (MCS) that can be supported on each of the codewords that may be mapped to the spatial streams, and the precoding matrix index, which is the precoding weights used on the aggregated set of antennas
The above determination of CSI is a straightforward extension of the existing procedures supported in LTE Release-8/9/10 specifications. However, the base station may need the flexibility to fall back to a single TP transmission (one set of colocated antennas/group of antennas) due to practical reasons like cell loading, traffic patterns etc., In which case, it will need access to CSI feedback related to individual TPs, otherwise referred to as per-TP feedback. So for example, if two TPs are considered, the total overhead could be three times the original overhead supported for a single TP feedback, since feedback needs to be supported assuming i) Joint transmission from two TPs ii) transmission from first TP and iii) transmission from second TP. Such three-fold increase is not desirable and further optimization is needed to achieve such operations at the base station with smaller increase in feedback overhead.
We address optimizing CSI feedback to support JP including non-coherent JT, Dynamic Point Selection (DPS) and possibly coherent JT.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions and/or relative positioning of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of various embodiments of the present invention. Also, common but well-understood elements that are useful or necessary in a commercially feasible embodiment are often not depicted in order to facilitate a less obstructed view of these various embodiments of the present invention. It will further be appreciated that certain actions and/or steps may be described or depicted in a particular order of occurrence while those skilled in the art will understand that such specificity with respect to sequence is not actually required. Those skilled in the art will further recognize that references to specific implementation embodiments such as “circuitry” may equally be accomplished via replacement with software instruction executions either on general purpose computing apparatus (for example, a CPU) or specialized processing apparatus (for example, a DSP). It will also be understood that the terms and expressions used herein have the ordinary technical meaning as is accorded to such terms and expressions by persons skilled in the technical field as set forth above except where different specific meanings have otherwise been set forth herein.